The desirability of providing carbon bodies having oxidation resistance is well-known. Carbonaceous materials such as monolithic carbon, graphite, and carbon-carbon composites of fibers in carbon have excellent strength-to-weight properties at high temperatures, e.g., 1400.degree. C. and higher, and are generally superior to conventional construction materials such as metals and superalloys at these temperatures. In addition, the mechanical strength of a carbon body increases as the temperature increases, whereas in conventional structural metals, the strength decreases with increased temperature.
The use of carbon bodies in high temperature applications has been limited due to the relatively high reactivity of carbon, principally with oxygen, at temperature above about 400-500.degree. C. which results in erosion of the carbon body due to the reaction between carbon and oxygen, yielding carbon monoxide and carbon dioxide. Accordingly, many attempts have been made to provide oxidation-resistant coatings for carbon bodies in order to permit their use in oxidizing environments and at elevated temperatures.
Major difficulties have been encountered in attempting to provide oxidation-resistant coatings on carbon bodies. One difficulty is the wide variation in the coefficient of expansion of various types of carbon bodies and differences in the coefficient of expansion between the carbon body and the coating material. Depending on the raw materials, the coefficient of expansion of the carbon body may be vastly different from that of the oxidation-resistant coating. The stresses that result from different coefficients of expansion between the coating and the underlying carbon body cause cracking or rupture of the coating, particularly when the part is subjected to thermal cycling, which allows oxygen to penetrate the coating and attach the underlying carbon body with resulting loss of structural integrity.
Surface porosity in the carbon body, which results from articles which are not fully densified, may cause pinholes to form in the coating during the coating process which also may result in the ability of oxygen to penetrate to the carbon surface. It has also been found that mechanical vibration, debris impingement, and the like may cause cracking of brittle protective coatings.
Successful resistance to high temperature oxidation may be achieved by the process disclosed in U.S. Pat. No. 4,515,860 which is incorporated herein by reference. The oxidation-resistant carbon body disclosed in this patent has thermochemically deposited thereon a silicon alloy coating containing one or more alloying elements selected from the group consisting of carbon, oxygen, aluminum, and nitrogen. The amount of silicon in the coating is in excess of the stoichiometric amount and the alloy coating has a noncolumnar grain distribution having substantially equiaxial grains of an average diameter of less than 1 micron. Because of the exceptionally fine grain size and even grain distribution in the coating, any cracks that may occur are extremely fine in width and form a mosaic pattern. The amount of silicon in excess of the stoichiometric amount fills in these fine cracks when the carbon body is heated to above the melting point of silicon, e.g., above 1410.degree. C., and reacts with any oxygen to form a glassy silicon oxide which acts as a filler sealing the cracks. This patent also contemplates, on an optional basis, particularly where lower temperature crack-resistance is desired, providing an intermediate boron layer. Boron reacts with oxygen to form a glassy boron oxide sealant and flows into any cracks that have formed. In commercial practice the carbon body is usually provided with a preliminary treatment in a mixture of chromic acid and sulfuric acid.
The oxidation resistance conferred by the coatings described in U.S. Pat. No. 4,515,860 provides significant superior characteristics as compared to the coatings of the prior art. Under some circumstances, however, particularly where severe temperature cycling occurs, the protection system may be inadequate to properly seal the cracking which occurs in the brittle coating such that the carbon body is subjected to oxidative attack.